Congenital osteopetrosis in mice has been reversed by the intravenous administration of cell suspensions prepared from the bone marrow and spleens of normal littermates. In addition the congenital bone disease has been induced in normal mice by the administration of osteopetrotic spleen cells. Cell labelling technics are being used currently to determine the distribution and fate of the donor cells in these experimental animals and histochemical and electron microscopic methods are being used to characterize the osteoclasts of animals receiving transplants of blood forming tissues.